LM-1

Ballast, bayonet base, 4 1/4" X 1 1/4", iron filament, in Hydrogen atmosphere, crinkled V-shaped filament; used for keeping current constant within fairly wide limits of generator voltage variation drawing no.RL-A-247.

NOTE

So I guess were talking about this, an Iron-hydrogen resistor witch has a wikipeda article

References

• 1920 signal corps storage catalogue